1. Field of the Invention
The invention relates to removing soot from exhaust gas of a combustion engine, more particularly from diesel exhaust gas.
2. Description of the Related Art
Generally, exhaust gas processing systems for diesel engines are comprised of a diesel oxidation catalyst (DOC), a diesel particulate matter filter (DPF), and a NOx converter. As for DOC, only an oxidation function of existing three-way catalysts is enhanced and poison resistance is increased. Since the DOC is technically reasonable, it has been mounted on diesel vehicles. However, the DOC has disadvantages in that in case of fuel with a great deal of sulfur as a component thereof, a large amount of particulate matters with sizes of 100 nm or less, which are harmful to human respiratory organs, are discharged at a rear end of the DOC, and the catalyst is poisoned by the sulfur component, resulting in degraded durability.
Table 1 below summarizes existing techniques for processing particulate matters.
TABLE 1Techniques for processing particulate matter of diesel exhaust gasProductPerformance andManufacturerNameTechniqueFeaturesJohnson-CRTOxidation catalyst +About 8,770 products onMattheyCeramic filtertestContinuousReduction of PM, COregeneration typeand HC to a level of 90%Applicable to largeReduction of NOx bydiesel enginesabout 3~8%Reduction of particles of10~250 nm among PMLow poison resistanceagainst sulfur is acritical obstacle topracticability.EngelhardDPXOxidation catalyst +About 5,000 products areCeramic filtermounted. (Europe/NorthContinuousAmerica/Taiwan)regeneration typeReduction of PM, COand HC by 70~90%,80% and 80%,respectively.Confirmed durability of20,000 hrs.Fuel with 350 ppm ofsulfur is usable.IbidenSiC-Silicone carbide7,300 products in use inDFPfilterEuropeAdditive, Catalyst,Cerium-based EolysElectrical heaterAdditive from RhodiaHeat is generated due tocombustion of unburnedhydrocarbon by a frontoxidation catalyst.Discharge by multiplefuel injection everyperiod of 400~500 kmForcible regeneration dueto raised gas temperatureAsh deposited in thefilter is washed withwater every 80,000 km.
As shown in Table 1, CRT (Continuously Regenerating Trap or Technologies) developed by Johnson-Matthey Plc. and DPX (Diesel Particulate Filter) developed by Engelhard Corporation comprise a suction part, a catalyst part, a filtering part and an exhaust part. In theses apparatuses, the filtering unit is DPF 101 employing a ceramic honeycomb monolith filter, and a diesel oxidation catalyst (DOC) 102 that is the catalyst part using noble metal is placed in front of the DPF 101 for capturing particulate matters contained in diesel exhaust gas flowing thereinto so as to react abundant oxygen and nitrogen oxides existing in the exhaust gas with each other, thereby producing nitrogen dioxide with superior oxidizing power, and oxidizing and removing the particulate matters captured by the DPF 101.
As for the principle that such type of DPF 101 captures the particulate matters, as shown in FIG. 2, exhaust gas flows into the DPF 101 through unit cells 103 with open upstream ends at an inlet of the filter, and downstream ends of the unit cells 103 with the open upstream ends are blocked by plugging 104 made of the same material as the filter, so that gaseous components of the exhaust gas pass through porous walls 105 into adjacent unit cells 106 with closed upstream ends and particulate matters which are solid components cannot pass through pores in the walls and thus are captured.
Meanwhile, if nitrogen dioxide (NO2) is sufficiently generated in the exhaust gas, CRT would have superior removal performance for the particulate matters 107. However, the generation of nitrogen dioxide is very sensitive to the temperature of the exhaust gas. That is, if the temperature of the exhaust gas is low, a noble metal catalyst may not be activated and thus nitric monoxide (NO) is not caused to react with oxygen, resulting in rapid reduction in a generation rate of nitrogen dioxide. Further, there is a disadvantage in that the noble metal catalyst is poisoned by sulfur component existing in diesel exhaust gas, resulting rapid degradation in its performance.
Moreover, as shown in FIG. 3, there has been developed a method of regenerating the DPF 101 through oxidation of nitric monoxide into nitrogen dioxide regardless of the temperature of exhaust gas and sulfur component by installing a plasma generator 111 in front of the DPF 101 to use strong oxidizing power of plasma, based on the same regeneration concept as described above. However, since such a method employing only plasma as a regeneration means requires a large amount of electric power to generate plasma, the method cannot be used in view of the capacity of a current battery 112. An inverter 113 for generation of plasma is expensive and has a large size.
In addition, as shown in FIG. 4, SiC-DPF developed by Ibiden Co., Ltd. is manufactured by changing cordierite ceramic used as the material of a conventional DPF into silicone carbide with superior mechanical strength, and is improved in that intermediate layers can be coated through increase in the porosity of walls of unit cells of the filter.
Therefore, there is a method of forcibly burning captured particulate matters using a heating means 121 such as a hot wire positioned in front of the DPF 101. However, this method has a problem in that since electric power consumption is very high in raising the temperature of exhaust gas containing a large amount of moisture up to 350° C. or higher that is a forcible combustion temperature for particulate matters, it is difficult to commercially implement the method using current batteries for diesel vehicles.
In addition to the aforementioned techniques for removing particulate matter or soot of diesel exhaust gas, there has been developed a method of adding an additive to fuel, wherein Ce or Fe is added as a catalyst to the fuel on the order of ppm. This method has advantages in that the metal catalyst contained in the fuel lowers the regeneration temperature of the DPF upon regeneration of the DPF and shortens a regeneration period of time. However, this method has disadvantages in that since the catalyst is metal, it increases back pressure of the DPF in cooperation with ash of engine oil and shortens the period of exchange of the DPF.
Meanwhile, as shown in FIG. 5, there is a system for processing exhaust gas using a photoreaction, which has been developed as an apparatus for removing harmful substances contained in exhaust gas of an internal combustion engine but does not reside in techniques for removing particulate matters contained in diesel exhaust gas as described above. This system has an advantage in that it can be operated with low electric power regardless of the temperature, air-fuel ratio and sulfur component of exhaust gas.
In the aforementioned system, contrary to the DPF for removing particulate matters contained in exhaust gas, a honeycomb monolith carrier 30 including unit cells 34 each of which is open at both ends thereof is coated with a photocatalyst and high voltage is applied to the both ends using electrodes 40 formed of metal nets or plates so that plasma can be generated within the unit cells 34 of the honeycomb monolith carrier 30, thereby exciting the photocatalyst and effectively reducing gas-phase harmful components existing in exhaust gas.
Although the conventional exhaust gas processing system using the photoreaction described above is effective for removal of gas-phase harmful components on the basis of strong oxidizing power and reducing power, the system has a problem in that since it takes more time for solid-phase components such as particulate matters contained in diesel exhaust gas to be oxidized as compared with gas-phase components, the solid-phase particulate matters cannot be sufficiently oxidized while passing though the unit cells with the open ends, and thus, it is impossible to remove the particulate matters.
Japanese Patent Laid-Open Publication Nos. (Sho)61-164014, (Hei)5-272326 etc. disclose apparatuses for processing diesel exhaust gas, wherein both ends of cells of a honeycomb monolith carrier are alternately plugged, the disclosures of which are considered to be a part herein.